Coating ferrous metals with molten aluminum



Patented Oct. 1, 1946" 2,408,623 I COATING FERROUS METALS wrrn MOLTEN ALUMINUM Harvey N. Gilbert,

to E. I. du Pont mington, Del.,

Niagara Falls, N. Y., assignor de Nemours & Company, Wila corporation of Delaware No Drawing. Application September 7, 1942, Serial No. 457,588

4 Claims. (Cl. 117-51) This invention relates to the production of corrosion-resistant coatings on iron or steel and more particularly to a process for coating iron or steel out in practice. The most successful aluminum coatings have been of the alloy type, e. g., coatings produced by heating iron with aluminum so as to produce an iron aluminum alloy coating.-

Other alloyed coatings such as coatings of zinc aluminum alloy, tin aluminum alloy and .the like have also been utilized. Various methods have been proposed for producing coatings of substantially pure aluminum but these are difficult to carry out and have not been used commercially to any great extent. Certain proposed methods utilize a molten aluminum bath in which steel articles may be immersed and in such processes various fluxes such as zinc chloride, tin chloride and the like are commercially used. Such fluxes have the disadvantage that they tend to react with the aluminum causing the formation of corresponding aluminum alloys and also the formation of volatile aluminum chloride.

The object of my invention is to provide an improved and simplified and more economical method for coating iron and steel articles with aluminum. Still other objectives will be apparent from the following description.

I have discovered the surprising fact that aluminum may be coated on iron or steel articles without the use of any flux if the article is simply cleaned to remove oxide and other commonly occurring surface impurities and then while wet with water it is immersed into a bath of molten aluminum. It is essential in practicing my invention that all of the surface desired to be coated is wet with water at the moment that the article is immersed in the aluminum bath.

In testing my invention I have compared it with the use of various known fluxing materials, including fused chlorides of zinc and tin, solutions of zinc, tin, and iron chlorides, strong alkaline solutions, fused alkalis, and solutions containing strong alkalis, and the like. In every instance I have found that the herein described process yields better results than when any of these fluxing materials are employed.

For a preferred modification of my invention I may take a steel article, e. g., hot rolled steel sheet and clean this with conventional aqueous cleaning media to remove adhering oxide scale. For example, I may pickle the steel sheet in a conventional acid pickling bath, then wash the pickled. steel with water to remove as much acid as possible, follow this with treatment in a dilute solution of alkaline material such as sodium carbonate or sodium cyanide to remove last traces of acid, then thoroughly wash the steel in water to remove any adhering alkali. The steel is then kept wet until it is ready to be coated with aluminum by keeping it under a spray of water or keeping it immersed in a tank of water. The wet steel sheet is then immersed in the bath of molten aluminum and directly removed .therefrom. If desired, the steel sheet may be left in the aluminum bath for any desired length of time but there is no advantage in this. Good results are obtained by removing the steel sheet substantially as quickly as practicable so that, e. g., it may remain in the aluminum bath for a period of say 2 to 30 seconds.

Various modifications of the invention will be apparent to those skilled in the art of coating metals without departing from the spirit and scope of my invention. For example, it is not essential that the metal be wet with absolutely pure water at the time it is immersed in the molten aluminum. While pure water is preferred, no great harm is done if the water contains up to as much as 5% by weight of water soluble impurities, which impurities may be acid, alkali, or

neutral in nature. The presence of insoluble impurities in the water in any amount does not particularly interfere with the adherence of the aluminum with .the wet steel articles. I prefer, however, to avoid the presence of such insoluble solid materials because they may cause some roughness in the coated sheet.

While it is preferable for reasons of economy and ease of operation, as well as to obtain best results, to immerse the steel in a molten aluminum bath, the invention is not restricted to this method of bringing the wet ferrous surface in contact with the molten aluminum. Other conventional methods for contacting articles with molten metals may be utilized,' for example spraying or pouring the molten metal on the ferrous surface, or casting the aluminum in a mold in contact with the ferrous metal.

My invention likewise is not restricted to coating the ferrous articles with absolutely pure aluminum. Various known aluminum alloys containing relatively small amounts of metals other than aluminum may be employed if desired. It is essential, however, that the coated metal contain at least by weight of aluminum.

The temperature of the aluminum coating bath a 3 may be maintained at any desired temperature above the melting point of aluminum. dep nding on the results desired. Ii it is desired to avoid the formation of any considerable amount of iron aluminum alloy in the coating, I prefer to maintain the bath at a temperature not too far above the melting point of aluminum, e. g., at a temperature of 670-750 C. At higher temperatures it will be apparent to those acquainted with coating steel with iron aluminum alloys. that such alloy coatings may be produced by my process. Also, the iron aluminum alloy coatings, of course, may be produced by first coating the articles with substantially pure aluminum by my process and then heating the aluminum coated articles, e. g., in a. muflle furnace, at a temperature suitable for the formation of'the iron aluminum alloy.

I claim:

1. A process for the coating of a steel structure with aluminum which comprises wetting the surface of said structure with water, and applying molten aluminumto the said substantially only water.

2. A process for the coating of a steel structure with aluminum which comprises wetting the surface of said structure with pure water. and applying molten aluminum to the said surface containing substantially only pure water.

3. A process for the coating of a steel structure with aluminum which comprises wetting the surface of said structure with water containing not to exceed 5% by weight of water-soluble impurity, and applying molten aluminum to the said surface containing substantially only water containing not to exceed 5% by weight of water-soluble impurity.

4. A process for the coating 01' a steel structure surface containing with aluminum which comprises pickling said structure with an acid medium to remove adhering oxide. washing the pickled article in water. and applying molten aluminum to the said sur face containing substantially only water.

HARVEY N. GILBERT. 

